I am installing a Fireplace Xtrodinair Flush insert into a Masonry fireplace. I will be running 4 foot of 6" stainless steel flex up and over the smoke shelf (after removing the Damper). The remaining flue will be 6" insulated rigid. The salesman indicated that it would be advisable to seal off the fire box from the chimney, using KAOWOOL or the like. In looking at KAOWOOL, it appears that it comes in 1" thick mats, and it could be a challenge getting it to stay in place. I have considered using fiberglass bats, but he indicates that they may melt (research indicates at 1100 f degrees). I see that mineral wool melts at 1800 F. Any thoughts on this? (possibly wrap the stainless flex in KAOWOOL, and then use fiberglass bats for the rest?